1. Field of the Invention
The present invention relates to an ink tank for retaining ink supplied to an ink jet recording head, and more particularly to an ink tank having an ink containing portion in which a negative pressure generating member, e.g., an ink absorber, for retaining ink, is contained.
2. Description of the Related Art
Hitherto, recording apparatuses for recording an image on recording media such as paper, cloth, plastic sheets and OHP sheets (referred to simply as "recording paper" hereinafter) have been proposed in the forms capable of mounting recording heads of various recording schemes such as wire dot, heat sensitive, heat transfer, and ink jet schemes.
Of those recording apparatuses, an ink jet recording apparatus has been employed and commercialized as output means of information processing systems, e.g., printers serving as output terminals of copying machines, facsimiles, electronic type writers, word processors and work stations, and handy or portable printers associated with personal computers, host computers, optical disk devices, video devices, etc.
Particularly, with recent popularity of the Internet and digital cameras, color ink jet recording apparatuses are becoming pervasive as means for outputting color images provided from the Internet and digital cameras.
In a color-adapted ink jet recording apparatus, a color image is formed by superimposing ink droplets in multiple colors ejected from recording heads. Color recording generally requires inks of three primary colors, i.e., yellow (Y), magenta (M) and cyan (C), or inks of four colors including black (B) in addition to the three primary colors, as well as three or four units of recording heads and ink cartridges corresponding to the inks.
Further, for the purpose of improving halftone representation of ink jet printers, it has become general to employ inks of six or seven colors including inks with light density (i.e., light yellow, light magenta and light cyan) in addition to the above inks of three or four colors.
Meanwhile, as an energy element for generating energy to eject ink through an ejection orifice of a recording head, there is known, e.g., a piezoelectric element using an electrical displacement transducer, an element for irradiating an electromagnetic wave, such as a laser beam, to generate heat and ejecting ink droplets under an action of the generated heat, or an element for heating a liquid with an electro-thermal transducer having a heat generating resistor.
In such an ink jet recording head, the printing speed has been increased by employing the multi-nozzle scheme and increasing the ejection frequency.
Also, an ink tank for supplying ink to the ink jet recording head is detachably attached to a recording apparatus in integral form with the recording head in many cases for the purpose of downsizing. Such a cartridge type ink tank integral with a recording head (including an structure in which the ink tank is always integral with the recording head and an structure in which the ink tank is separable from the recording head) is required to generate a predetermined negative pressure relative to the recording head. One method of generating a predetermined negative pressure is realized by inserting, in the ink tank, an ink absorber which is then impregnated with ink. The ink absorber is formed of a porous member such as a urethane foam, or a felt using fibers of a thermoplastic resin such as polypropylene.
The use of the above-mentioned multi-color system in six or seven colors naturally increases the kinds of inks used. In particular, when ink tanks containing inks of respective colors are constructed in the separate form allowing the tanks to be replaced independently for each color, six or seven ink tanks have to be arranged side by side. Therefore, it is desired from a restriction in overall size that the width of each ink tank is as small as possible.
Moreover, employing the multi-nozzle scheme and increasing the ejection frequency to achieve a higher printing speed, as described above, increases a flow rate of the ink per unit time. Correspondingly, an ink supply flow rate required for the ink tank is also increased. This may cause such a risk in a tank having a narrow width that a sufficient sectional area of a flow passage is not secured and resistance of the flow passage against the ink flow is too increased to supply ink to the recording head reliably. In the recording head, a flow passage is enlarged corresponding to the increased ink flow rate, and the diameter of a joint portion of the recording head with the ink tank is also increased. This has resulted in a difficulty in mounting an ink tank having a narrow width to the joint portion. In an ink tank having a narrow width, an ink absorber inserted in the ink tank also has the form having a narrow width. Further, increasing the amount of ink contained in an ink tank renders the ink tank to have a flat thin shape. In such a case, walls of the ink tank are required to have a greater thickness relative to the tank width for securely providing sufficient strength of a tank housing (case). If so, the ink flow in a direction from the ink absorber toward an ink supply port is restricted, and hence insufficient supply of ink may occur depending on the condition in use. Another problem is that because the absorber has a narrow width, the ink may be left in contact areas between inner wall surfaces of the tank and the absorber.
On the other hand, focusing an attention to an ink absorber inserted in the ink tank, it is preferable for improving the efficiency in use of ink contained in the tank that the ink absorber be inserted in the ink tank under compression increasing gradually toward the ink supply port jointed to the recording head.
However, even if the absorber inserted in the ink tank can be given a satisfactory coarse/dense relation adjusted with good accuracy, such a state is difficult to maintain for a long period of time, and the satisfactory coarse/dense relation may be lost due to a restoring force caused by elasticity of the absorber itself and an impact applied externally in the event of a drop or the like. Further, for the similar reasons, it may occur somewhere that the absorber is not held in good close contact with the inner wall of the ink tank.